Solvation and stabilization of palladium nanoparticles in phosphonium-based ionic liquids: a combined infrared spectroscopic and density functional theory study

Abstract

Analysis of infrared spectra of palladium nanoparticles (NPs) immersed in the tri-tert-butyl-R-phosphonium-based ionic liquids (ILs) demonstrates that both cations and anions of the ILs interact with the NPs. According to quantum-chemical simulations of these interactions, the binding energy of anions to the Pd₆ cluster, taken as a minimal-size model of the NPs, increases from ∼6 to ∼27 kcal mol⁻¹ in the order [PF₆]⁻ ≈ [BF₄]⁻ < [Tf₂N]⁻ < [OTf]⁻ < [Br]⁻ ≪ [TFA]⁻. In contrast, the binding energy for all types of the [Bu^t₃PR]⁺ cations slightly varies at about ∼22 kcal mol⁻¹ only moderately depending on the choice of the R moiety (n-pentyl, 2-hydroxyethyl, 2-methoxyethyl, 2-ethoxy-2-oxoethyl). As a result, the energies of interaction between a Pd₆ cluster and various ion pairs, formed by the abovementioned counter-ions, follow the order found for the anions and vary from ∼24 to ∼47 kcal mol⁻¹. These values are smaller than the energy of addition of a Pd atom to a Pd_n cluster (∼58 kcal mol⁻¹), which suggests kinetic stabilization of the NPs in phosphonium-based ILs rather than thermodynamic stabilization. The results are qualitatively similar to the trends found earlier for interactions between palladium clusters and components of imidazolium-based ILs, in spite of much larger contributions of the London dispersion forces to the binding of the [Bu^t₃PR]⁺ cations to the cluster (up to 80%) relative to the case of 1-R-3-methylimidazolium cations (up to 40%).